As our previous study has indicated, perineuronal proteoglycans in the adult mouse brain are associated with some collagenous molecules which can be stained with Gömöri's ammoniacal silver and are resistant to hyaluronidase digestion. The present study demonstrated that these molecules are thoroughly digested with collagenase, and suggests that they represent a hyaluronic acid-binding domain of the ligand proteoglycans connecting the perineuronal proteoglycans and nerve cell surface glycoproteins.